1. Field of the invention
The present invention relates to a method for forming a bump on a pad electrode of a semiconductor device. More specifically, the present invention relates to a method for forming a bump, which can remove a portion of an organic film by using an excimer laser and thereby form an opening.
2. Description of the related art
Conventional methods for forming a bump on a pad electrode of a semiconductor device include a metal evaporation method, an electroplating method, a stud-bumping method, an adhesive applying method, etc. However, these methods for forming a bump may destroy a passivation film because the passivation film is suffered with an excessive force by covering pinholes on the passivation film of the semiconductor wafer and by deformation of the bump with an unbalanced force. Thus, in order to prevent the destruction of the passivation film, a polyimide film has been used.
In addition, a photoresist for plating is used in forming a bump by using the electroplating method. FIG. 7 shows a process chart of a conventional method for forming a bump. The conventional method is the electroplating method using the polyimide film and the photoresist film for plating. In the illustrated process, a photo-insensitive polyimide is used. FIGS. 8(a)-8(j) show cross-sectional views of a pad electrode portion of a semiconductor wafer in the major steps shown in FIG. 7.
The conventional method for forming a bump using an electroplating method will be described with reference to these figures.
As shown in FIG. 8(a), a semiconductor wafer 1 which has a passivation film 2 and an aluminum electrode 3 thereon, is washed (S1), and then a polyimide film 10 is coated on the passivation film 2 and the aluminum electrode 3 (S2) as shown in FIG. 8(b). Thereafter, the polyimide film 10 is dried by pre-baking step(S3) and post-baking step(S4) for the polyimide film 10.
Next, a photoresist for etching(RS) is coated on the polyimide film 10 and thereby a photoresist film for etching 15 is formed (S5) as shown in FIG. 8(c). Then, common steps of the electroplating method, such as pre-baking step(S6), exposure step(S7) and developing step(S8) of the photoresist film, are carried out. Consequently, the portion of the photoresist film for etching 15 is removed as shown in FIG. 8(d) and an opening 16 is formed.
Next, post-baking step (S9) of the photoresist film for etching 15 is carried out, whereby the photoresist film 15 is dried. Thereafter, an exposed portion of the polyimide film 10 at the opening 16 of the photoresist film for plating is removed by using hydrazine in the etching process(S10), whereby an opening 11 of the polyimide as shown in FIG. 8(e) is formed. Thereafter, the photoresist film 15 is exposed with an ultra visible light, etc., whereby the photoresist film 15 is removed by a resist removing agent (S11). Thereafter, aluminum, chrome and copper is deposited onto the polyimide film 10 and the opening 11 of the polyimide by vacuum evaporation, whereby an under bump metallurgy(UBM) 4 for the bump is formed (S12) as shown in FIG. 8(f). The under bump metallurgy 4 also has the function of the common electrode of electroplating.
Next, a photoresist is coated on the under bump metallurgy 4, whereby a photoresist film for plating 20 is formed (S13) as shown in FIG. 8(g). Then, pre-baking step(S14), exposure step(S15) and developing step(S16) of the photoresist for plating 20 are carried out. Consequently, the portion of the photoresist for plating 20 is removed as shown in FIG. 8(h), whereby an opening 21 of the photoresist for plating is formed. Then, post-baking step of the photoresist for plating 20 is carried out (S17).
Next, the above processed semiconductor wafer is attached to a plating unit (no illustration) (S18), then plating of copper 30 (S19) and plating of solder 40 (S20) are carried out with pre-determined thickness respectively. Thereafter, the photoresist for plating 20 is removed by a resist removing agent (S21) and the unnecessary under bump metallurgy 4 is removed by etching with a solder 40 as an etching mask (S22). Thereafter, a flux is applied on plated solder 40 (S23), and then the solder 40 is melted in a reflow furnace under a nitrogen atmosphere, whereby a spherical shaped solder bump 41 is obtained as shown in FIG. 8(j). Thereafter, an inspection is carried out and a process for forming a solder bump 41 is completed.
However, this method for forming a solder bump by using an electroplating method has the problems as follows.
First, after the post-baking step of the polyimide, the residue (scum) of polyimide (i.e. ultra thin film) is left on the aluminum electrode in the periphery of the opening 11 of the polyimide film. The residue of polyimide can not be removed in the developing step. As a result, the opening 11 of the polyimide with a predetermined size can not be obtained and the quality of the bump becomes worse. Especially, when the size of the opening 11 of the polyimide is 40 .mu.m or less, the residue of polyimide can not be ignored and thereby a plasma dry etching is needed in order to remove the residue of polyimide.
Second, six steps for forming the opening 11 of the polyimide, such as the photoresist coating step (S5) through the photoresist post-baking step (S9) and the photoresist removing step, and two steps for forming the opening 21 of the photoresist for plating, such as, the photoresist for plating pre-baking step (S14) and the photoresist for plating exposure step, are necessary and therefore it takes a long time to complete the process.
Third, because of the forgoing first and second problems, the process time becomes long and the process cost becomes also expensive.